1. Field of the Invention
This invention is in the field of printing and pertains more particularly to a method and apparatus for varying the density of a printed image.
1. The Prior Art
It is well known that the density of a printed image is dependent largely upon the quantum of ink delivered to the printing roll of a printing press.
In printing systems heretofore known substantial difficulties have been encountered in achieving a desired printing density. Density may vary across the printed image where certain elements of the image require higher concentrations of ink than others.
Desirably, the printing roll during each cycle of operation has deposited thereon a repeatable and precisely determined quantity of ink. Where, however, certain portions of the image, in the course of printing, result in large quantities of ink being removed from the roll supplying ink to the printing roll, a larger quantum of ink must be replaced on the supply roll to such areas than to other areas of the supply roll where little ink has been removed.
Numerous systems have been suggested for assuring the application of repeatable quantities of ink to the printing cylinder. Certain such systems employ a train of intermediate rolls between the ink supply and the printing cylinder together with a multiplicity of ink keys transversely located across the width of the roll train at various positions. By proper adjustment of the keys, there may ultimately be obtained a printed image having a desired density across the entire width. However, adjustment of each such key affects an interaction with the adjustment of the other keys, with the result that the services of a highly skilled operator are necessary for manipulation of the keys, and even such skilled operator will waste many hundreds of pages before a balancing is effected.
Where a desired balance is achieved, if the printing density is to be modified a still further series of manual adjustments must be effected. The criticality of such adjustments is most apparent in color printing operations where the tone or color of the ultimate printed image is dependent upon the density of each of the respective color layers which is over-printed.
In accordance with certain inking systems, known for instance as flexographic or flexo printing, uniform density across the width of the image may be easily achieved. Such systems employ a so-called anilox roll, i.e. a roll having a multiplicity of minute recessed cells in the surface which, by virtue of the geometry of the cells, draws a more or less precise amount of ink from the ink fountain or other supply during each revolution.
The anilox roll will deliver a precisely equal quantum of ink to the printing cylinder. This is so because, if larger quantities of ink are removed from the anilox roll, the depleted cells will be free to receive greater amounts of ink from the source than will those cells from which only small quantities of ink have been removed.
A disadvantage of the flexo system resides in the fact that it is difficult to vary within small gradations the over-all density of the image, a procedure which is particularly desirable in color printing. This is so because the long train of rolls which permit density adjustment in conventional printing procedures is eliminated in the interests of obtaining uniform side-to-side density in the manner noted above.
In a flexo printing system, density variations are effected by removing a given anilox roll and substituting a different roll having different cell capacities. Such method of controlling density provides only limited shades or nuances, i.e. one for each roll. In addition, anilox rolls are expensive.
An alternate proposed method for providing variable density and across-the-page density consistency is suggested in U.S. Pat. No. 4127,067.
In accordance with the method of such patent, a uniform ink film is obtained by indenting a first roll into the surface of a second roll to deform said surface while rotating the indenting roll at very high speeds in the order of thousands of rpm in counter-direction to the indented roll, utilizing the ink film as a lubricant.
The device of this patent has proven commercially unsuccessful and impracticable due to its complexity and to the rapid wearing away of the indented roll. Moreover, if the ink film should be removed even for an instant, the surface of the indented roll is destroyed, necessitating replacement.